here's a map of chicago from the city's department of environment. these red built-up pockets are where the temperature is hotter throughout the city. or look at this satellite view of phoenix's night temperatures, where yellow areas are the hottest. the difference between phoenix and a neighboring rural town is about 11 degrees. and making all that worse is the fact that scientists have seen a rise in surface temperature in the last century worldwide. by looking at data from urban and rural areas, land and sea, they've been able to account for local impacts including weather stations near heat retaining structures. that is, they know that chicago is hotter-- and not just because of all those buildings. thomas peterson is a climatologist with the national oceanic and atmospheric administration. >> we look at temperatures at rural stations and we look at temperatures at urban stations. we see if they are showing the same thing in the long-term trend-- variability and change-- and they are showing the same thing in our record, both in the u.s. record a